This invention relates to an oxygen sensor which utilizes an oxygen ion conductive solid electrolyte in the form of a tube closed at one end and has a construction particularly suitable for detecting oxygen concentration in exhaust gas of automotive engines.
An oxygen sensor which has a layer of an oxygen ion conductive solid electrolyte such as stabilized zirconia and operates on the principle of an oxygen concentration cell is well known. This type of oxygen sensor is suitable for detecting oxygen concentration in hot gases, particularly in exhaust gas of internal combustion engines for, chiefly, automotive use as an element of a feedback control system for controlling the air-fuel ratio of a combustible mixture fed to the engines.
In practical applications of this type of oxygen sensors to exhaust systems of internal combustion engines, the solid electrolyte layer in most cases is formed into the shape of a tube which is closed at one end for convenience of attachement to, for example, exhaust pipes for the engines and exposure of one side of the solid electrolyte layer to the exhaust gas and the opposite side to the atmospheric air which serves as a reference gas. The outer and inner surfaces of the solid electrolyte tube are coated with porous (permeable to gases) and electron conductive layers of a metal such as platinum respectively as anode and cathode electrodes of the oxygen concentration cell. This electrolyte tube is tightly inserted into a tubular metal shell such that a closed end portion of the tube protrudes from the shell. This shell has on its outside an attachment means such as screw threads for attachement of the sensor to, for example, a boss formed on an exhaust pipe. Besides, the shell serves as an anode conductor. A tubular metal member is inserted into the solid electrolyte tube from its open end to serve both as a cathode conductor and as an air admission conduit. One of a pair of leads is connected to the anode conductor while the other is connected to either the shell or a cap member (having an air inlet opening) attached to the shell.
When the closed end portion of the electrolyte tube of this sensor is exposed to an exhaust gas stream in the exhaust pipe and the inside of the tube is exposed to the atmospheric air, an electromotive force is developed across the anode and cathode electrodes. The magnitude of this electromotive force varies according to the concentration of oxygen in the exhaust gas relative to the oxygen concentration in air.
The concentration cell of this sensor has a considerably high internal impedance particularly when the electrolyte tube is not sufficiently heated. For example, the internal impedance is above 10 M.OMEGA. at cold starting of the engine. To accurately pick up the developed electromotive force even when the sensor is operated at low temperatures, it is important to maintain very high insulation resistances both between the anode and cathode electrodes and between the leads.
A problem encountered by this oxygen sensor, particularly when the sensor is used in automobiles, is splashing of water on the sensor. Since the sensor in an automobile is attached to either the exhaust pipe or an exhaust manifold and the inside of the electrolyte tube is exposed to the atmosphere, the sensor is frequently splashed with water particularly at the open end portion of the electrolyte tube. The splashing of water causes the aforementioned insulation resistances to lower. Furthermore, the electrolyte tube is liable to crack when splashed with water in a heated state. (The temperature of the electrolyte tube, even at its open end portion located outside of the exhaust pipe, reaches to about 300.degree. C. during a continuous operation of the engine.)
To protect the electrolyte tube against splashing of water, it has been proposed to cover the open end of the electrolyte tube with a ceramic disk supported by a cap member attached to the shell of the sensor. However, the provision of this ceramic disk cannot perfectly prevent the inside of the electrolyte tube and/or the joints between the leads and the conductors from being wetted with water, because the cap member has an air inlet opening.